This invention relates to an exhaust gas recirculation (EGR) control system for an internal combustion engine.
EGR systems are well known in the art, which essentially feed a portion of the exhaust gas back to the intake side of an engine to reduce the level of noxious and harmful nitrous oxide (NOx) emissions. Since the flow of the recirculated exhaust gas affects the performance and fuel consumption, etc., of the engine, however, in addition to reducing the NOx emission level, the exhaust gas feedback must be accurately controlled according to the operating conditions of the engine to optimally balance these offsetting features or characteristics.
After prolonged use an EGR control valve is subject to the buildup of carbon and other exhaust gas contaminants, which renders its control function inaccurate. U.S. Pat. No. 4,614,175 seeks to overcome this problem by providing a feedback control system having a sensor for detecting the oxygen concentration in the intake air/recirculated exhaust gas mixture, the exhaust gas feedback then being controlled in accordance with the sensor output since the oxygen concentration is proportional to the mixing ratio of the exhaust gas.
A suitable oxygen pump type sensor for such a system is described in U.S. Pat. No. 4,578,172. The sensor of this patent employs oxygen-ion-conductive solid electrolyte plates, and is provided with an oxygen pump element, an electric current detecting element disposed opposite the pump element with a gap between the two, and an atmospheric air chamber for establishing a reference for the pump element. Since the pump element pumps out oxygen in the gap, a difference in oxygen concentration between the mixed gas in the gap and the atmospheric air in the chamber is established such that the detecting element generates an electrical output signal proportional to such difference, and thus proportional to the inflow rate of the recirculated exhaust gas.
Although the pressure in the intake duct in which the mixed gas is flowing changes according to the operating conditions of the engine, the pressure in the reference air chamber does not change because of its atmospheric venting. This pressure discrepancy causes sensing errors which adversely affect the performance of the EGR system.